STOICHIOMETRY

Stoichiometry is the science of using balanced chemical
equations to determine exact amounts of chemicals needed
or produced in a chemical reaction.
Determining Ratios in Balanced
Chemical Reactions

What is the balanced chemical reaction for the reaction
between nitrogen gas (N2) and hydrogen gas (H2) to produce
ammonia (NH3)?

This means that for every one mole of N2 you have (28.02 g) you
need to have 3 mole of H2 (3 × 2.02 g = 6.06 g)…
(34.08 g of reactants)

You make 2 mole of NH3 ( 14.01 + 3.03 = 17.04 * 2 = 34.08 g)
Mole Ratios

These are what we call mole ratios

Example 1: If you had 5 mol of N2, how many mol of H2 would
you need? How many mol of NH3 would you make?
Example Problem 2: Calculations
Involving Mass of Reactants

Propane, C3H8(g), is a gas that is commonly used in
barbeques. Calculate the mass of oxygen gas, O2, that is
needed to burn 15 g of propane. (Produces CO2 and H2O)
Example Problem 2: Calculations
involving Numbers of Entities and
Mass

How many molecules of oxygen are produced from the
decomposition of 12 g of water into its elements?
 Recall:
O2 and H2 are produced
Practice Problems:
1.
Bauxite ore contains aluminum oxide, Al2O3, which is
decomposed using electricity to produce aluminum metal and
oxygen gas (O2). What masse of aluminum metal can be
produced from 125 g of aluminum oxide? How many grams of
O2 are produced?
2.
Potassium metal, K(s), reacts with hydrochloric acid, HCl(aq), to
produce aqueous potassium chloride and hydrogen gas, H2.
How many grams of potassium are required to produce 5.00 g
of hydrogen gas?
3.
Potassium chlorate, KClO3, decomposes when heated to form
solid potassium chloride and oxygen gas. How many grams of
KClO3 must decompose to produce 0.96 g of O2?
Limiting Reactant, Excess Reactant
and Percent Yield

In the real world, you never have perfect amounts of every
reactant you need…
Example

In the synthesis of water, we require exactly 2 mol of H2 and
exactly 1 mol O2 to create exactly 2 mol of H2O…

What would happen if we have a 2 mol H2 and 8 mol of O2?
 Would
this change the amount f H2O that would be
created?

No…
 There
 O2
would be 7mol of O2 left over.
is an excess reagent; H2 is limiting
Balanced
Equation
2 H2
O2
2H2O
Moles
2
1
2
Molar Mass
2.02 g/mol
32.00 g/mol
18.02 g/mol
Mass
4.04 g
32.00 g
36.04 g
Balanced
Equation
2 H2
O2
2H2O
Moles
2
8
2
Molar Mass
2.02 g/mol
32.00 g/mol
18.02 g/mol
Mass (Needed)
4.04 g
32.00 g
Mass (Used
Up/Made)
4.04 g
32.00 g
Left Over
0.00 g
224.00 g
36.04

A chemical reaction will stop once any one of the reactants
runs out.
 This
reactant is known as the limiting reagent (limiting
reactant)
 Any
other reactant is an excess reagent
Example 1:
Table Salt, BaCL, can be formed by the reaction of sodium metal
and chlorine gas. A reaction mixture contains 45.9 g of sodium
and 142.0 g of chlorine. Calculate the mass of sodium chloride
that is produced.
Example 2:

Determine the mass of carbon monoxide that is produced
when 32.1 g of methane, CH4 undergoes incomplete
combustion with 160.0 g of oxygen (products: CO and H2O)
Example 3:

Phosphorus, P4, reacts with chlorine gas to produce solid
phosphorus pentachloride as the only product. Determine the
mas of phosphorus pentachloride that is produced from a
reaction between 123.88 g of phosphorus and 950.00 g of
chlorine.
Percentage Yield

Yield: the quantity of product produced in a chemical reaction

Actual Yield: the quantity of product that is actually produced
in a chemical reaction

Theoretical Yield: the quantity of product calculated from a
balanced chemical equation (using stoichiometry)

There are many reasons why the amount of a product
predicted for a reaction may not actually form
 Reactions
may not go to completion
 Some
of the reactant may be impure
 There
may be competing side reactions
 It
may be difficult to collect the product

Example 1:

Iron is produced from its ore, hematite, Fe2O3, by heating
hematite with carbon monoxide in a blast furnace. If 635 g of
iron is obtained from 1150 g of hematite, what is the
percentage yield of iron?
Practice 1:

The most common ore of arsenic, FeSAs, can be heated to
produce arsenic and iron(II) sulfide. When 250 g of the ore
was processed industrially, 95.3 kg of arsenic was obtained.
Calculate the yield of arsenic.
Homework…

You have a test on Monday, November 24 at 6:30 pm, sharp.

I strongly recommend you study for it.